1. Field of the Invention
The present invention relates to an epoxy resin composition for encapsulating a semiconductor device, and more particularly to an epoxy resin composition for encapsulating a semiconductor device which provides a cured product which is capable of maintaining the moisture resistance and heat resistance of the epoxy resin and which has a low modulus of elasticity, a low coefficient of thermal expansion, and a high glass transition temperature.
2. Description of the Related Art
In recent years, semiconductor devices have tended to be provided with a larger chip area and an encapsulating resin of a lesser thickness. At the same time, there has been a trend toward a greater degree of integration. Accordingly, if a semiconductor device is encapsulated with a conventional epoxy resin composition, fatal flaws in a semiconductor part can occur, such as a crack in the chip, the cutting of a bonding wire, the sliding of an aluminum wire, and the cracking of the encapsulating resin. This is because the conventional semiconductor encapsulating epoxy resin has been developed from the perspective of heat resistance and moisture resistance, such that the cured product lacks flexibility and an unwanted large stress to the device can result.
Generally, there are two possible measures for reducing the stress to a semiconductor-encapsulating epoxy resin: one is to reduce the thermal strain by decreasing the coefficient of thermal expansion of the resin, and the other is to reduce the stress due to thermal strain by decreasing the modulus of elasticity. In addition, when viewed from the aspect of maintaining the heat resistance and moisture resistance and expanding a temperature region having a small thermal strain, it is necessary to set the glass transition temperature to a high level. A flexibilizer may be added as a method of reducing the stress to a low level. Conventional flexibilizers include, for instance, long-chain alkylene polyamine, polyoxy alkylene glycol, and bisphenol A-type diglycidyl ether having long-chain alkylene oxide. With the method of reducing the modulus of elasticity of a resin by using such a flexibilizer, there is a drawback in that the drop in the glass transition temperature is large, so that the heat resistance and humidity resistance decline. (Refer to Japanese Patent Publication Nos. 59-8718, 59-30820, and 59-226066.)
Meanwhile, to obtain flexibilizers in which the decline in humidity resistance and glass transition temperature is small, elastomer modified flexibilizers have been devised which are obtained from polybutadiene having functional groups capable of reacting with an epoxy resin or phenol resin at both ends, or from a copolymer of butadiene and acrylonitrile. (Refer to Japanese Patent Publication Nos. 58-108220, 58-174416, 58-184204, 62-9248, 59-113021, and 59-58024. However, the aforementioned elastomer flexibilizers have a problem in that their flexibilizing effect disappears over time as the unsaturated bonds in the elastomer are oxidized at high temperature and deteriorate as a result.
In addition, a method is also known in which a silicone resin or a silicone rubber which has a low modulus of elasticity is dispersed in the resin and is an outstanding flexibilizer in terms of electrical properties and thermal stability at a high temperature (refer to Japanese Patent Publication Nos. 62-84147 and 56-4647). However, since a silicone resin has poor adhesive properties with respect to a metal (frame and the like), and since a silicone rubber has a weak interfacial strength with respect to an epoxy matrix, the moisture permeability of the cured product is large, so that there is the problem that the product lacks reliability in that its moisture resistance and mechanical strength are low.